Selective carbon dioxide reduction catalyzed by single metal sites on carbon nitride under visible light irradiation

ABSTRACT

A composition of a photocatalyst, a method of manufacturing the photocatalyst, and a method of chemically reducing carbon dioxide to carbon monoxide using the photocatalyst under visible-light irradiation is provided. The photocatalyst comprises a transition metal ion and graphitic carbon nitride and includes single metal sites on carbon nitride. Under visible light, the metal sites that are coordinated to nitrogen atoms get activated, without the use of additional ligands, to catalyze the reduction of carbon dioxide to selectively produce carbon monoxide. The photocatalytic reduction of carbon dioxide to carbon monoxide is highly efficient, resulting a turnover number of more than 800 for carbon monoxide production in 2 hours. The composition is useful in converting carbon dioxide into useful chemicals and carbon-based fuels. A functional model of molecular catalysts for efficient carbon dioxide reduction is also present.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 62/856,443, filed Jun. 3, 2019, the disclosure of which is incorporated by reference herein in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to photocatalysts made of earth-abundant elements and, in particular, to photocatalysts for reducing carbon dioxide into energy-rich fuels under visible light.

BACKGROUND

A photocatalyst is a material that absorbs light to pump electrons to a higher energy level and subsequently transfers electrons to facilitate a chemical reaction to occur. In many chemical reactions, metal-ligand complexes have been used as catalysts. However, they are often expensive and difficult to prepare.

SUMMARY

In one aspect, a photocatalyst is provided. The photocatalyst may include a graphitic carbon nitride coordinated with atomically dispersed Co^(n+) in absence of additional ligands. The Co^(n+) may form coordinate bonds with nitrogen atoms in the graphitic carbon nitride where the nitrogen atoms may maintain a flat framework and may form a plane within the graphitic carbon nitride. The Co^(n+) may be positioned outside the plane. In one embodiment, Co^(n+) may be Co²⁺. Co²⁺ may be present in any effective amount, for example, at a concentration between 0.004 and 0.430 μmol/mg of the photocatalyst. Co²⁺ may be uniformly distributed on the graphitic carbon nitride. The molar ratio of Co²⁺ to cobalt oxide in the photocatalyst may be greater than 1000. The graphitic carbon nitride may be planar and may also include carbon doping. Co^(n+) may be off the plane formed by the nitrogen atoms by a distance less than 0.5 Angstrom. Co^(n+) may form coordinate bonds with four nitrogen atoms on the graphitic carbon nitride. The photocatalyst may include earth-abundant elements.

In another aspect, a method for manufacturing a photocatalyst is provided. The method includes preparing a mixture of graphitic carbon nitride and a cobalt salt in a polar solvent and forming a Co^(n+)-carbon nitride complex. The Co^(n+)-carbon nitride complex may include a graphitic carbon nitride having single metal sites that may be directly coordinated with Co^(n+), and Co^(n+) may form coordinate bonds with nitrogen atoms in the graphitic carbon nitride. The nitrogen atoms may maintain a flat framework and may form a plane within the graphitic carbon nitride. The Co^(n+) may be positioned outside the plane. The graphitic carbon nitride may include carbon doping. In one embodiment, Co^(n+) is Co²⁺. The complex may have a Co²⁺ concentration between 0.004 and 0.430 μmol/mg of the complex. The mixture may further include triethylamine. The polar solvent may include acetonitrile. The polar solvent may include a volume ratio of acetonitrile to triethylamine of from 100:1 to 150:1. The cobalt salt may include a cobalt halide. The cobalt salt may be a cobalt dichloride. The method for manufacturing the photocatalyst may include heating the mixture and heating may include using microwave radiation. The mixture may be heated to at least 80° C. for at least 2 hours. The method may also include stirring in presence of a dispersant. The method may further include using X-ray absorption spectroscopy to confirm a single metal ion site structure.

In another aspect, a method for chemically reducing carbon dioxide is provided. The method may include dispersing a photocatalyst in a polar solvent, and the photocatalyst may include a graphitic carbon nitride including single metal sites directly coordinated with metal ions. The method may also include introducing carbon dioxide to the dispersion to provide a carbon dioxide containing dispersion, irradiating the carbon dioxide containing dispersion with visible light, and reducing at least some of the carbon dioxide to carbon monoxide. The dispersion may include an electron donor. The electron donor may be triethanolamine. The polar solvent may be acetonitrile. The dispersion may include a volume ratio of acetonitrile to triethanolamine of from 2:1 to 10:1. The graphitic carbon nitride may include carbon doping. The nitrogen atoms in the carbon nitride may maintain a flat framework in a plane and the metal ions may form coordinative bonds with the nitrogen atoms and are positioned outside the plane. The metal ions may be transition metal ions. In some embodiments, the metal ions may be cobalt ions. In one embodiment, the metal ions may be Co²⁺, and Co²⁺ may be off the plane formed by the nitrogen atoms by a distance less than 0.5 Angstrom. Co²⁺ may be present at a concentration between 0.004 and 0.430 μmol/mg of graphitic carbon nitride. The photocatalyst may reduce carbon dioxide to yield carbon monoxide. In the method, visible light may be provided by solar radiation or by a halogen lamp. Visible light may include photons with wavelengths between 350 nm and 800 nm. In some embodiments, visible light may contain photons with wavelengths between 420 nm and 650 nm. The method may also include recycling the photocatalyst. The morphology of the photocatalyst may remain unchanged after recycling the photocatalyst. Carbon dioxide may be selectively reduced to CO with a turnover number of carbon monoxide between 1 and 250, where the turnover number is a ratio of moles of carbon monoxide to moles of cobalt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a triazine structure of graphitic carbon nitride (C₃N₄), and FIG. 1B illustrates a heptazine structure of C₃N₄.

FIG. 2 is a schematic illustration of the photocatalytic reduction of CO₂ to CO by single Co²⁺ sites on C₃N₄.

FIG. 3 illustrates amounts of cobalt loadings on Co²⁺—C₃N₄ prepared in presence (solid cycle) and absence (open cycle) of triethylamine (TEA) using different amounts of CoCl₂ in the synthesis.

FIG. 4 illustrates X-ray powder diffraction (XRD) pattern of bare C₃N₄. Bare C₃N₄ denotes C₃N₄ without any cobalt or other kinds of loadings.

FIG. 5A illustrates optical spectra of bare C₃N₄ and Co²⁺—C₃N₄ with different amounts of cobalt loadings. FIG. 5B illustrates an optical spectrum of CoO_(x)/SiO₂ in the powder form. The peaks between 500 nm and 700 nm are associated with cobalt oxides.

FIG. 6 illustrates the structure of Co-cyclam, a molecular catalyst that contains a macrocyclic ligand (cyclam).

FIG. 7 illustrates infrared spectra of bare C₃N₄ and Co²⁺—C₃N₄ with different amounts of cobalt loadings.

FIG. 8A-F illustrate (A) TEM and (B) SEM images of bare C₃N₄; (C) TEM image, (D) SEM image of Co²⁺—C₃N₄ with a cobalt loading of 0.345 μmol/mg; and elemental distribution of (E) nitrogen atoms and (F) Co of Co²⁺—C₃N₄ with a cobalt loading of 0.345 μmol/mg.

FIG. 9 illustrates X-ray photoelectron spectra (XPS) of bare C₃N₄ and Co²⁺—C₃N₄ with different amounts of cobalt loadings.

FIG. 10 illustrates normalized Co K-edge X-ray absorption near edge structure (XANES) spectra of Co-cyclam, high-Co²⁺—C₃N₄, low-Co²⁺—C₃N₄, CoO_(x)/SiO₂, CoO, and Co₃O₄.

FIG. 11 illustrates a close comparison of normalized Co K-edge XANES spectra of Co-cyclam, high-Co²⁺—C₃N₄, low-Co²⁺—C₃N₄, CoO_(x)/SiO₂, CoO, and Co₃O₄.

FIG. 12 illustrates O 1s XPS of bare C₃N₄ and Co²⁺—C₃N₄ with different amounts of cobalt loadings.

FIG. 13 illustrates a Fourier transform magnitude of k²-weighted Co K-edge extended X-ray absorption fine structure (EXAFS) spectra of Co-cyclam, high-Co²⁺—C₃N₄, low-Co²⁺—C₃N₄, CoO_(x)/SiO₂, CoO, and Co₃O₄.

FIG. 14 illustrates output spectra of the halogen lamp, used in this study, with and without a 420-nm long-pass optical filter.

FIG. 15 illustrates the amounts of CO produced in CO₂ reduction using (a) 1 mg Co²⁺—C₃N₄ with a cobalt loading of 0.054 μmol/mg; (b) a dispersion of 0.054 μmol CoCl₂ and 1 mg C₃N₄; and (c) 1 mg bare C₃N₄ (light intensity 200 mW/cm²).

FIG. 16 illustrates the amounts of CO produced after CO₂ reduction for 2 h using Co²⁺—C₃N₄ with different amounts of cobalt loadings.

FIG. 17A illustrates turnover numbers (TONs) for CO and H₂ produced as a function of reaction time by a Co²⁺—C₃N₄ sample. FIG. 17B illustrates TONs of CO production after CO₂ reduction for 2 h using Co²⁺—C₃N₄ with different amounts of cobalt loadings. The open cycle represents a Co²⁺—C₃N₄ sample prepared in the absence of TEA. Close cycles represent Co²⁺—C₃N₄ samples prepared in presence of TEA.

FIG. 18 illustrates TONs of CO produced by CO₂ reduction under visible-light irradiation (λ>420 nm, light intensity 200 mW/cm²) using 1 mg Co²⁺—C₃N₄ with a cobalt loading of 0.004 μmol/mg prepared in the absence of TEA. The insert illustrates the infrared spectrum of gaseous ¹³CO produced in the reduction of ¹³CO₂.

FIG. 19 illustrates selectivity of Co²⁺—C₃N₄ with different amounts of cobalt loadings toward CO production after CO₂ reduction for 2 h. The open cycle represents a Co²⁺—C₃N₄ sample prepared in the absence of TEA. Close cycles represent Co²⁺—C₃N₄ samples prepared in presence of TEA.

FIG. 20 illustrates CO production upon CO₂ reduction for five 2-hour cycles using 10 mg Co²⁺—C₃N₄ with a cobalt loading of 0.128 μmol/mg in 4.0 mL acetonitrile containing triethanolamine (TEOA) (light intensity 60 mW/cm²).

FIG. 21A-B illustrate SEM images of Co²⁺—C₃N₄ with a cobalt loading of 0.004 μmol/mg: (FIG. 21A) before, and (FIG. 21B) after photocatalysis for 30 h. FIG. 21C-D illustrate SEM images of Co²⁺—C₃N₄ with a cobalt loading of 0.128 μmol/mg. (FIG. 21C) before and (FIG. 21D) after photocatalysis for five 2-hour cycles.

FIG. 22 illustrates UV-vis spectra of various C-doped C₃N₄ samples, “C(x)-C₃N₄”, with x=0, 5, 20, 55, 150 and 400 from bottom to top.

FIG. 23 illustrates TONs for CO produced after photocatalytic CO₂ reduction for 2 hours using Co²⁺ on C(x)-C₃N₄ samples with x=0, 5, 20, 55, 150, and 400 from left to right.

FIG. 24 illustrates Fourier transform magnitude of k²-weighted Co K-edge EXAFS spectra of Co²⁺ on C(x)-C₃N₄ samples with x=0, 5, 20, 55, and 150 from bottom to top.

FIG. 25 illustrates a structure of potential molecular catalysts for CO₂ reduction where R=H or NH₂ and M²⁺=Co²⁺ or Ni²⁺.

Various aspects of at least one example are discussed below with reference to the accompanying figure, which is not intended to be drawn to scale. The figure is included to provide an illustration and a further understanding of the various aspects and examples, and are incorporated in and constitute a part of this specification, but are not intended to limit the scope of the disclosure. The drawings, together with the remainder of the specification, serve to explain principles and operations of the described and claimed aspects and examples. For purposes of clarity, not every component may be labeled in every figure.

GENERAL OVERVIEW

Described herein is a graphitic carbon nitride including single metal sites that form coordinate bonds with transition metal ions (M^(n+)). These carbon nitride transition metal structures are hereafter referred to as M^(n+)-C₃N₄. In M^(n+)-C₃N₄, the graphitic carbon nitride can include single metal sites forming coordinate bonds with transition metal ions in absence of any additional ligands. The M^(n+)-C₃N₄ is capable of catalyzing the photoreduction of CO₂ under visible light. M^(n+)-C₃N₄ reduces CO₂ to yield CO in the presence of an electron donor. M^(n+)-C₃N₄ is highly active and selectively reduces CO₂ to CO. In one set of embodiments, the transition metal ion can be Co^(n+). In specific embodiments, Co^(n+) can be Co²⁺. Co²⁺ can form co-ordinate bonds with nitrogen atoms in the graphitic carbon nitride to form Co²⁺—C₃N₄ that includes single metal sites. The Co²⁺—C₃N₄ is an efficient photocatalyst under visible-light irradiation and can exhibit a turnover number, the ratio of moles of CO produced to moles of Co²⁺ in the photocatalyst, of more than 800 after photocatalysis for 2 hours.

DETAILED DESCRIPTION

Carbon nitrides are compounds of carbon and nitrogen. They are covalent network compounds that include beta carbon nitrides and graphitic carbon nitride. They may be void of other elements or void of other elements except for hydrogen. The beta carbon nitrides are solids with a formula β-C₃N₄, and can have a hardness of greater than that of diamond. Graphitic carbon nitrides are also solids and have a formula g-C₃N₄, or simply C₃N₄. It can have two major substructures based on triazine units, as shown in FIG. 1A, and heptazine units, as shown in FIG. 1 i, which, depending on reaction conditions, exhibit different degrees of condensation, different properties and varied reactivities.

The structure of C₃N₄, in general, is similar to that of graphene, but with a carbon lattice that has been partially substituted with nitrogen atoms in a regular fashion. The structure of C₃N₄, in general, is planar. However, in some embodiments, C₃N₄ structure can include planar sheets, or corrugated sheets, or both. In some embodiments, C₃N₄ can be fully polymerized. In some other embodiments, C₃N₄ can be partially polymerized. Synthesized C₃N₄ materials typically contain hydrogen atoms. The polymeric C₃N₄ structure can be highly ordered, with some hydrogen atoms at the edges of C₃N₄ flakes, in the forms of —NH₂ and —OH groups.

C₃N₄ can be prepared by several different methods including by polymerization of cyanamide, dicyandiamide or melamine. In one set of embodiments, C₃N₄ can be prepared by pyrolysis of urea. In particular, a desired amount of urea can be calcined in a muffle furnace at 600° C. for 4 hours (with a ramp rate 5° C./min) to prepare C₃N₄.

The calcination temperature and its duration can also vary. In some embodiments, C₃N₄ can be prepared by calcining urea in a muffle furnace at a temperature less than 1000° C., less than 800° C. or less than 700° C. In some other embodiments, C₃N₄ can be prepared by calcining urea in a muffle furnace at a temperature greater than 300° C., greater than 400° C., greater than 500° C. or greater than 600° C. Similarly, in some embodiments, C₃N₄ can be prepared by calcining urea in a muffle furnace at these temperatures for more than 2 hours, more than 3 hours, more than 4 hours or more than 6 hours.

C₃N₄ can form complexes with transition metal ions. Transition metals are typically elements of groups 4-11 of the periodic table. They display a typical chemistry including formation of a large range of complex ions in various oxidation states and can exhibit catalytic properties either as the element or as ions (or both). Transition metals with catalytic properties that may be useful in various embodiments include, for example, one or more of Ni, Fe, Cu, Pt, Pd, and Co.

Cobalt is one of the abundant metals found in the Earth's crust. In fact, cobalt comprises about 0.0029% of the Earth's crust and, compared to some other transition metals, is relatively inexpensive to use as catalyst. Cobalt exists in many oxidation states ranging from −3 to +5. However, cobalt compounds in the Co²⁺ and Co³¹ states are the most common. Cobalt compounds include cobalt oxides, cobalt sulfides, and cobalt halides. The cobalt oxides includes cobalt(II) oxide or cobalt monoxide (CoO), cobalt(II, III) oxide (Co₃O₄), and cobalt(III) oxide (CO₂O₃). The cobalt sulfides include cobalt(II) sulfides such as CoS₂, and cobalt(III) sulfide (Co₂S₃). Four dihalides of cobalt(II) include cobalt(II) fluoride (CoF₂), cobalt(II) chloride (CoCl₂), cobalt(II) bromide (CBr₂), and cobalt(II) iodide (CoI₂). The halides exist in both anhydrous and hydrated forms.

Transition metal ions may be capable of forming coordination complexes with C₃N₄. In one embodiment, C₃N₄ can form a complex with a Pt ion. In another embodiment, C₃N₄ can form a complex with a Pd ion. In yet another embodiment, C₃N₄ can form a complex with a cobalt ion. The cobalt ion can form a metal-ligand complex with C₃N₄ (Co²⁺-carbon nitride complex) in its different oxidation states including, but not limited to, Co⁺, Co²⁺ and Co³⁺. In one embodiment, C₃N₄ forms a complex with Co²⁺ to produce Co²⁺—C₃N₄.

Co²⁺—C₃N₄ can be formed by loading cobalt on C₃N₄. In one embodiment, C₃N₄ can be mixed with a cobalt dichloride in a polar solvent to form a mixture or a dispersion. In some embodiments, the mixture/dispersion can also include a base. The mixture can then be subsequently stirred for an hour and heated in a microwave reactor at about 80° C. for about 120 min. In some embodiments, mixing can be achieved by stirring the dispersion comprising C₃N₄ into the solution containing cobalt ions. Stirring can be achieved by a repeated manual stirring process or by using a magnetic stirrer. In some embodiments, a dispersant, for example a polymer, can also be added to the dispersion to maintain the dispersion for longer time.

In some embodiments, the polar solvent may be a polar aprotic solvent such as, for example, acetonitrile, acetone, dimethyl sulfoxide (DMSO), or N, N-Dimethylformamide (DMF). However, in some other embodiments, the polar solvent may be a polar protic solvent, and it may include, for example, one or more of water, ammonia, t-butanol, n-propanol, ethanol, methanol, and acetic acid. In one embodiment, the polar solvent is acetonitrile.

In some embodiments, the base can be a Schiff base. The Schiff base can be a symmetric base or an asymmetric base. In some embodiments, the symmetric base may be ethylenediamine, diethylenetriamine or triethylenetetramine. In one embodiment, the base is triethylamine.

In various embodiments, the volume ratio of acetonitrile to triethylamine can vary. In one embodiment, the volume ratio of acetonitrile to triethylamine can be 115.4:1. In other embodiments, the volume ratio of acetonitrile to triethylamine can be 100:1. In another embodiment, the volume ratio of acetonitrile to triethylamine can be 150:1. In yet other embodiments, the volume ratio of acetonitrile to triethylamine can be from 50:1 to 300:1, from 50:1 to 100:1, from 100:1 to 150:1, or from 150:1 to 300:1.

While C₃N₄ and cobalt dichloride can react at room temperature to form Co²⁺—C₃N₄, the reaction can be accelerated by heating. Heating can be done either by using a traditional heating source, for example, a hot plate/stove or by using a microwave oven. In some embodiments, the temperature of the dispersion being reacted can be between 50° C. and 100° C., between 60° C. and 100° C. or between 70° C. and 90° C. Similarly, duration of the reaction time can also vary. In some embodiments, the dispersion can be heated for more than 30 minutes, more than 60 minutes or more than 120 minutes.

Loading efficiency of cobalt on Co²⁺—C₃N₄ depends on many factors including the presence or absence of a promotor in the dispersion. Bases such as triethylamine (TEA) promote coordination of cobalt ions with nitrogen atoms in C₃N₄ to form Co²⁺—C₃N₄, and therefore higher cobalt loadings on Co²⁺—C₃N₄ can be achieved in presence of a base. The base helps (1) Co²⁺ to coordinate cobalt with the available nitrogen atoms (“N atoms”) of C₃N₄, and (2) helps Co²⁺ deposit on C₃N₄ as CoO_(x) at higher cobalt loadings. However, coordination between Co²⁺ and nitrogen can be achieved even without TEA but it may take longer. In the absence of a base, CoO_(x) does not form easily on C₃N₄, even at higher Co²⁺ concentrations.

In various embodiments, the amount of cobalt in the Co²⁺—C₃N₄ product can be in the range of 0 to 0.05, 0.05 to 0.1, 0.1 to 0.15, 0.15 to 0.2, 0.2 to 0.25, 0.25 to 0.3, 0.3 to 0.35, 0.35 to 0.4, 0.4 to 0.45, 0.45 to 0.5, 0.5 to 0.6, 0.6 to 0.7, 0.7 to 0.8, 0.8 to 0.9, 0.9 to 1, 1 to 1.2, 1.2 to 1.4, 1.4 to 1.6, 1.6. to 1.8, and 1.8 to 1.10 μmol/mg of Co²⁺—C₃N₄. In some embodiments, the amount of cobalt in Co²⁺—C₃N₄ can be less than 10, less than 5, less than 2.5, less than 2, less than 1, less than 0.5, less than 0.25, or less than 0.1 μmol/mg of Co²⁺—C₃N₄. In the same and other embodiments, the amount of cobalt in Co²⁺—C₃N₄ can be more than 0.1, more than 0.5, more than 1, more than 2, more than 3, more than 4, more than 5, or more than 10 μmol/mg of Co²⁺—C₃N₄.

Extended X-ray absorption fine structure (EXAFS) analysis of Co²⁺—C₃N₄ samples confirm that each Co²⁺ forms four coordinate bonds with four N atoms at edge sites. At low cobalt loadings (e.g., below approximately 0.01 mmol Co²⁺ per gram Co²⁺—C₃N₄), greater than 90% of the cobalt exists as Co²⁺ coordinated with N atoms on the C₃N₄. At high cobalt loadings, some cobalt can form CoO_(x) clusters as at higher cobalt loadings available N atoms for coordination may already be occupied by Co²⁺. In some embodiments, at both low and high cobalt loadings, Co²⁺ can be uniformly distributed. Uniform distribution of Co²⁺ means that the cobalt ions are not concentrated in any specific area of the carbon nitride and are evenly distributed across the material.

The Co^(n+) can be positioned outside the plane formed by the nitrogen atoms. For example, the cobalt ion may be outside the plane of the coordinating nitrogen atoms by a distance of greater than 0.1 Angstrom or greater than 0.2 Angstrom and/or less than 0.5 Angstrom.

In Co²⁺—C₃N₄, the molar ratio of Co²⁺ to CoO_(x) may depend on the amount of cobalt reacted with C₃N₄. At low cobalt loading, the molar ratio of Co²⁺ to CoO_(x) can be high. In contrast, at higher loadings, the molar ratio of Co²⁺ to CoO_(x) can be lower. In various embodiments, the molar ratio of Co²⁺ to CoO_(x) may be greater than 1, greater than 10, greater than 100, greater than 1000, greater than 10000. In some embodiments, the molar ratio of Co²⁺ to CoO_(x) may be less than 1, less than 10, less than 100, less than 1000 or less than 10000.

Co²⁺—C₃N₄ is capable of reducing CO₂ to CO under visible light. FIG. 2 is a schematic presentation of the photocatalytic reduction of CO₂ to CO mediated by single Co²⁺ sites on C₃N₄. C₃N₄ can harvest visible light and can activate single Co²⁺ sites without the use of additional ligands and can reduce CO₂ to CO selectively. C doping of C₃N₄ is considered to be important for the superior CO₂-reduction activity using Co²⁺—C₃N₄, as discussed in more detail with reference to FIGS. 22-25. In absence of single Co²⁺ sites, C₃N₄ has been shown to be unable to catalyze the reduction of CO₂ to CO. As used herein, visible light has a wavelength in the range of 400 nm to 800 nm. In some embodiments, the visible light source can irradiate Co²⁺—C₃N₄ in a range of wavelengths between 420 and 650 nm. Sources of visible light can be either solar radiation or an artificial source such as lamp, for example, a halogen lamp.

The photocatalytic reduction of CO₂ to CO mediated by Co²⁺—C₃N₄ is believed to be a multi-electron transfer process, as shown below:

CO₂+2H⁺+2e→CO+H₂O

In the photocatalytic reduction of CO₂ to CO, an electron donor such as TEOA can be used to provide electrons and protons needed for the reaction. In this process, TEOA is transformed to oxidized products, while CO₂ is reduced to CO. A halogen lamp can be used to provide photons with wavelength greater than 350 nm as the driving force for the reactions to occur.

Depending on the number of electrons CO₂ receives during the photocatalytic reduction process, a number of products can be formed, for example, CO or formic acid (if 2 electrons are received); formaldehyde (if 4 electrons are received); methanol (if 6 are electrons received); and methane (if 8 electrons are received). Besides these carbonaceous products, H₂ is another competing side product in photocatalysis. In one embodiment where Co²⁺—C₃N₄ is used as a photocatalyst in reduction of CO₂, CO is the only carbonaceous product that is formed in measurable quantities. CO accounts for at least 75% of the product and H₂ is formed as a side product (accounting for less than 25% of total product). Therefore, Co²⁺—C₃N₄ can catalyze CO₂ reduction to selectively produce CO. As used herein, a product is selectively produced if no other carbonaceous product is formed upon reducing CO₂.

To test the photocatalytic CO₂ reduction properties of Co²⁺—C₃N₄, Co²⁺—C₃N₄ can be dispersed in a polar solvent containing an electron donor in a quartz test tube, and CO₂ can be bubbled into the dispersion (in the dark) and followed by irradiation with a visible light source. In one embodiment, the photocatalytic reduction of CO₂ yields CO as major products. Substantial saturation can be achieved by bubbling CO₂ into the dispersion at a rate of about 0.01 standard cubic feet per hour (SCFH), or 5 mL/min or 0.2 mmol/min for a period of 20 minutes. In some embodiments, CO₂ can be introduced in its highly pure form (99.999% purity). In other embodiments, CO₂ can be introduced in the form of air or a gaseous mixture such as combustion gases.

In some embodiments, the polar solvent may be a polar aprotic or a polar protic solvent, as described above. In one embodiment, the polar solvent is acetonitrile.

In various embodiments, the sacrificial electron donor may include, for example, an aliphatic amine or an aromatic amine or benzyl-dihydronicotinamide (BNAH) or dimethylphenylbenzimidazoline (BIH) or ascorbic acid or an oxalate or a thiol or mixtures thereof. In one embodiment, the sacrificial electron donor is TEOA.

In various embodiments, the volume ratio of polar solvent to electron donor can vary. In one embodiment, the volume ratio of polar solvent to electron donor can be 4:1. In some other embodiments, the volume ratio of polar solvent to electron donor can be 2:1. In another embodiment, the volume ratio of polar solvent to electron donor can be 10:1. In yet other embodiments, the volume ratio of polar solvent to electron donor can be from 2:1 to 10:1, from 1:1 to 8:1, or from 2:1 to 6:1.

Co²⁺—C₃N₄ can be reused or recycled many times in the photocatalytic reduction of CO₂ to CO process. The photocatalyst, Co²⁺—C₃N₄, may be collected from the dispersion after the reduction of CO₂. Collection techniques can include, for example, filtration and centrifugation, and the material can be further used with no significant loss of photocatalytic activity after washing with, for instance, acetonitrile.

EXPERIMENTS AND RESULTS Preparation OF C₃N₄

C₃N₄ was prepared by pyrolysis of urea (98% purity). Twenty grams of urea was weighed into a covered crucible and was calcined in a muffle furnace at 600° C. for 4 hours (with a ramp rate 5° C./min) to prepare C₃N₄. Further experiments indicate that this C₃N₄ sample contains doped carbon atoms, e.g., carbon atoms that are not covalently bound in the graphitic structure or that replace some of the N atoms. Co²⁺—C₃N₄ without carbon doping has been shown to be incapable of reducing CO₂ to CO, as described below.

Preparation of Carbon-Doped Carbon Nitride (C(x)-C₃N₄)

C(x)-C₃N₄ samples were also prepared to examine the effect of C doping on the activity of C₃N₄ prepared by the pyrolysis of urea. In order to prepare C(x)-C₃N₄, different amounts of dextrose and 20 g urea (purity>99.6%) were uniformly mixed and thoroughly ground, and then the mixture was transferred into a covered crucible and calcined in a muffle furnace at 600° C. for 4 h (ramp rate 5° C./min).

Preparation OF Co²⁺—C₃N₄

A Co²⁺—C₃N₄ sample was formed by coordinating cobalt ions with C₃N₄. 100 mg C₃N₄ was mixed with a desired amount of CoCl₂ in 7.5 mL acetonitrile. The amount of CoCl₂ depended on the desired loading of Co²⁺ in the complex. Subsequently, 65 L of TEA was added to the mixture, and then the mixture was stirred for an hour. A capped reaction vessel containing the mixture was placed in a single-mode microwave reactor (CEM Discover), and was heated to 80° C. for 120 min. After 120 min, the resulting precipitate was recovered from the capped reaction vessel by centrifugation and was washed twice with chloroform, methanol and acetonitrile sequentially. The precipitate obtained was dried at room temperature, and the resulting precipitate was denoted as “Co²⁺—C₃N₄”. Following the same method, Co²⁺—C₃N₄ samples were synthesized in the absence of TEA. Single C²⁺ sites were also deposited on C(x)-C₃N₄ samples following the same procedure.

A control sample, CoO_(x)/SiO₂ with cobalt loading 0.254 μmol/mg and denoted as “CoO_(x)/SiO₂,” was synthesized using 100 mg SiO₂ and 5.0 mg CoCl₂ in the presence of TEA.

A standard cobalt complex, Co-cyclam, a molecular catalyst, was synthesized using a method as described here. Cobalt(II) chloride (1.3 g) dissolved in 30 mL methanol was added to a solution of the 1,4,8,11-tetraazacyclotetradecane (cyclam) (2.0 g) in 20 mL methanol to form a brown-colored solution, and then air was bubbled through the brown solution for 1 hr. One hour later, concentrated hydrochloric acid (3 ml) was added which resulted in a change in color of the solution from brown to deep green. Air was bubbled through the solution for an additional hour, and then the solution was filtered and evaporated to dryness. The green residue was recrystallized using a minimum volume of water at 80° C., and the green needle crystals formed were filtered off and washed with acetone and ether sequentially.

Preparation of Low-Co²⁺—C₃N₄ and High-Co²⁺—C₃N₄ Complexes

In Co²⁺—C₃N₄, the cobalt loading can be varied by reacting different amounts of CoCl₂. In presence of TEA, the cobalt concentration in Co²⁺—C₃N₄ was varied between 0.004 and 0.430 μmol of Co²⁺ per mg of Co²⁺—C₃N₄. However, when the reaction was carried out in the absence of TEA, the highest cobalt loading successfully achieved was only 0.016 μmol/mg even when an excess of CoCl₂ was used in the reaction. A Co²⁺—C₃N₄ complex loaded with 0.016 μmol of Co²⁺ per mg of Co²⁺—C₃N₄ is referred to herein as “low-Co²⁺—C₃N₄,” whereas a Co²⁺—C₃N₄ loaded with 0.430 μmol of Co²⁺ per mg of Co²⁺—C₃N₄ is referred to herein as “high-Co²⁺—C₃N₄.”

FIG. 3 shows the actual cobalt loadings in Co²⁺—C₃N₄ prepared in the presence and absence of TEA at different amounts of CoCl₂ used in the synthesis. In the presence of TEA, as the amount of CoCl₂ in the reaction mixture increases, the concentration of cobalt in Co²⁺—C₃N₄ also linearly increases. In fact, approximately 45% of CoCl₂ used in reaction mixture in the presence of TEA was successfully loaded on C₃N₄. However, in the absence of TEA, the concentration of cobalt in Co²⁺—C₃N₄ does not increase in proportion to the increase in the amount of CoCl₂.

Material Characterization

Elemental analysis was conducted by acid digestion, followed by quantification using a Varian Vista AX induced coupled plasma atomic emission spectrometer. X-ray diffraction (XRD) patterns of powder samples were collected on a Rigaku XDS 2000 diffractometer using nickel-filtered Cu Kα radiation (λ=1.5418 Å). Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) were collected on an Amray 3300FE field emission SEM with PGT Imix-PC microanalysis system. Transmission electron microscopy (TEM) images were obtained on a Zeiss/LEO 922 Omega system. X-ray photoelectron spectra (XPS) were collected on a Kratos Axis HS XPS system. UV-visible spectra were obtained on a Cary 50 Bio spectrophotometer. A Barrelino diffuse reflectance probe was used to collect UV-visible spectra of powder samples using BaSO4 as a standard. Transmission FTIR spectra were collected on a Thermo Nicolet iS10 FTIR spectrometer. Results are shown in FIGS. 4, 5A-B, 7, 8A-F, and 9, and are described below.

FIG. 4 shows XRD pattern of C₃N₄ prepared by pyrolysis of urea as described above.

FIG. 5A shows the optical spectra of C₃N₄ with 0, 0.004, 0.054, and 0.345 μmol of Co^(n+) loadings per mg of Co²⁺—C₃N₄. The optical spectrum of bare C₃N₄ prepared from 98% urea, that is, Co²⁺—C₃N₄ with no Co²⁺ or any other loading, exhibits significant photoresponse in the visible region (400-800 nm). Loading of Co²⁺ on C₃N₄ does not result in any notable change in the optical spectrum of C₃N₄. FIG. 5B shows the optical spectra of CoO_(x)/SiO₂, a control sample. The peaks between 500 and 700 nm are associated with cobalt oxides.

FIG. 7 shows the infrared spectra of C₃N₄ with 0, 0.004, 0.054, and 0.345 μmol of Co²⁺ loadings per mg of Co²⁺—C₃N₄. The infrared spectra of Co²⁺—C₃N₄ samples are almost identical to that of bare C₃N₄, confirming that the cobalt loading does not affect the structure of C₃N₄.

FIGS. 8A-D show TEM and SEM images of bare C₃N₄ and Co²⁺—C₃N₄. There is no discernible difference in the morphologies of bare C₃N₄ and Co²⁺—C₃N₄ as seen in their TEM and SEM images. That indicates that Co²⁺ on C₃N₄ does not result in any measurable morphological changes to the C₃N₄. This confirms that the process, as described above, used for loading cobalt on C₃N₄ does not affect C₃N₄ morphology.

FIGS. 8E and 3F show elemental distribution of nitrogen atoms and cobalt atoms, respectively, in a Co²⁺—C₃N₄ sample with a cobalt loading of 0.345 μmol/mg of Co²⁺—C₃N₄. The microscopic mapping of Co²⁺—C₃N₄ reveals that Co²⁺ is uniformly distributed on C₃N₄ within Co²⁺—C₃N₄ structure.

FIG. 9 shows XPS spectra of four Co²⁺—C₃N₄ samples with 0, 0.004, 0.054, and 0.345 μmol of Co²⁺ loadings per mg of Co²⁺—C₃N₄. The presence of a peak at 781 eV in the spectra of cobalt loaded samples confirms the presence of Co²⁺ sites in Co²⁺—C₃N₄. However, when the cobalt loading is zero (i.e. in the bare C₃N₄ sample), the cobalt peak is absent. This confirms that the Co²⁺—C₃N₄ includes Co²⁺ sites.

X-ray absorption spectra at Co K-edge were taken at the beamline 7-BM (QAS) of NSLS-II at Brookhaven National Laboratory. Si (111) double crystal was used as monochromator and detuned 30% to reduce harmonics. The 15-cm long ion chambers, which were filled with 100% N₂, were used for detection of incident and transmitted beams, and a passivated implanted planar silicon (PIPS) detector was used for detection of fluorescence from the sample. The beam size was 1.4 mm (vertical)×6 mm (horizontal). Co-cyclam, high-Co²⁺—C₃N₄ and CoO_(x)/SiO₂ samples were measured in transmission mode, and the low-Co²⁺—C₃N₄ was measured in fluorescence mode. CoO_(x)/SiO₂ was deposited on tape and the other samples were made into 13 mm diameter pellets. At least three scans were measured for each sample. All measurements were performed in ambient atmosphere at room temperature and the samples were held in 45 geometry. A Co foil was placed between the two detectors downstream from the sample and measured simultaneously with the sample as reference for energy alignment. The existing data for Co oxides were aligned with the samples' spectra using their respective reference foil spectra. Results are shown in FIGS. 10, 11, and 13 are described below.

FIG. 10 shows normalized Co K-edge XANES spectra of low-Co²⁺—C₃N₄ and high-Co²⁺—C₃N₄. FIG. 10 also shows Co K-edge XANES spectra of some other cobalt control samples including CoO, Co₃O₄, CoO_(x)/SiO₂, and Co-cyclam. FIG. 11 shows a close comparison of normalized Co K-edge XANES spectra shown in FIG. 10. A close comparison of Co K-edge XANES spectra of low-Co²⁺—C₃N₄ and high-Co²⁺—C₃N₄ with that of other cobalt samples reveals that cobalt is in oxidation state +2 in both low-Co²⁺—C₃N₄ and high-Co²⁺—C₃N₄ samples, and the oxidation state of cobalt in Co²⁺—C₃N₄ is +2. The XANES spectra also indicate that the cobalt species in CoO_(x)/SiO₂ and Co-cyclam are at higher oxidation states than the cobalt in low-Co²⁺—C₃N₄ and high-Co²⁺—C₃N₄.

FIG. 10 further reveals that Co²⁺ in Co²⁺—C₃N₄ is not in the same plane as the plane defined by the flat framework of nitrogen atoms in C₃N₄. The flat framework of nitrogen atoms implies that nitrogen atoms in C₃N₄ occupy a single plane. The results further indicate that the Co²⁺ ions are off the plane of nitrogen atoms by a distance less than 0.5 Angstrom. This is confirmed by the absence of a shoulder feature along the rising edge around 7715 eV of the normalized Co K-edge XANES spectra of Co²⁺—C₃N₄. Co K-edge XANES spectra of Co-porphyrins, a cobalt complex having four-coordinate square-planar structure, typically shows a shoulder feature along the rising edge around 7715 eV. Therefore, it is clear that Co²⁺ in Co²⁺—C₃N₄ is not in the same plane formed by the atoms making up the ligand. This is in contrast to the cobalt in a Co-porphyrin where the cobalt and porphyrins share a common plane.

FIG. 10 shows that the shape of Co K-edge XANES spectra of low-Co²⁺—C₃N₄ is different from those of CoO and Co₃O₄, and this is likely due to the coordination of Co²⁺ with nitrogen atoms of C₃N₄ in low-Co²⁺—C₃N₄. In contrast, the spectral shapes of high-Co²⁺—C₃N₄ and CoO_(x)/SiO₂ are similar to that of the CoO, indicating the presence of CoO in both high-Co²⁺—C₃N₄ and CoO_(x)/SiO₂.

FIG. 12 shows the oxygen 1s XPS of bare C₃N₄ and Co²⁺—C₃N₄ with 0, 0.004, 0.054, and 0.345 μmol of Co²⁺ loadings per mg of Co²⁺—C₃N₄. In the spectra of Co²⁺—C₃N₄ samples, O 1s peaks that are associated with O²⁻ are seen between 528 eV and 530 eV. This is likely due to the presence of OH⁻ bound to Co²⁺ as well as CoO in the sample with a cobalt loading of 0.345 μmol of Co²⁺ loadings per mg of Co²⁺—C₃N₄. The O 1s XPS spectra also confirms that the presence of CoO in high-Co²⁺—C₃N₄.

FIG. 13 shows Fourier transform magnitude of k²-weighted Co K-edge EXAFS spectra of low-Co²⁺—C₃N₄, high-Co²⁺—C₃N₄, CoO, Co₃O₄, CoO_(x)/SiO₂, and Co-cyclam. The spectra of both high-Co²⁺—C₃N₄ and low-Co²⁺—C₃N₄ exhibit a peak around 1.55 Å (uncorrected for the photoelectron phase shift), which is similar to that of Co-cyclam. This further indicates the coordination of Co²⁺ with nitrogen atoms of C₃N₄ in Co²⁺—C₃N₄ complexes.

FIG. 13 demonstrates that there is no Co—O—Co (i.e. cobalt-oxygen-cobalt) contribution in low-Co²⁺—C₃N₄, indicating that Co²⁺ exists as isolated but is coordinated with four nitrogen atoms of C₃N₄. In low-Co²⁺—C₃N₄, the cobalt may be unassociated with oxygen atoms, halide atoms or any atoms other than those that comprise the carbon nitride. In contrast, there is a Co—O—Co contribution in high-Co²⁺—C₃N₄ indicating that high-Co²⁺—C₃N₄ sample contains some cobalt oxides which do not contribute in CO₂ reduction to CO, as described later. This confirms that cobalt in low-Co²⁺—C₃N₄ exists as isolated single sites. This result highlights the importance of X-ray absorption spectroscopy in establishing that the metal ions in the photocatalyst are atomically dispersed.

Photocatalytic Reduction of Co₂ Under Visible Light

Co²⁺—C₃N₄ catalyzes the reduction of carbon dioxide under visible light to carbon monoxide. In order to test the photocatalytic CO₂ reduction properties of Co²⁺—C₃N₄, 1 mg of Co²⁺—C₃N₄, was dispersed in a 4.0 mL acetonitrile solution containing triethanolamine (TEOA) (acetonitrile:TEOA=4:1 v/v) in a quartz test tube. Prior to photocatalytic testing, the reaction solution was bubbled with CO₂ (99.999%, Airgas) at 5 mL/min in the dark for 20 min. The reaction solution was then irradiated with a halogen lamp equipped with a water filter. FIG. 14 shows the output spectra of the halogen lamp used in this study with (shown in dotted trace) and without (shown in solid trace) a 420-nm long-pass optical filter. In some tests, a long-pass (λ>420 nm) optical filter was used with the lamp. Light intensity on the reaction solution was fixed at 200 mW/cm². The head space above the reaction solution was sampled with a gas-tight syringe at different time intervals for product analysis using an Agilent 7820 gas chromatograph equipped with a thermal conductivity (TCD) detector and a 60/80 Carboxen-1000 packed column (Supelco). In isotope experiments, ¹³CO₂ (Sigma-Aldrich, carbon-13 99%) was introduced into a reaction solution containing 4 mg Co²⁺—C₃N₄ after degassing the solution through freeze-pump-thaw cycling. The head space above the reaction solution was sampled and analyzed in a gas cell with the Thermo Nicolet iS10 FTIR spectrometer. Results are shown in FIGS. 15A-B and 16, and are described below.

FIG. 15 shows amounts of CO produced in different experimental conditions by using: (a) 1 mg Co²⁺—C₃N₄ with a cobalt loading of 0.054 μmol/mg of Co²⁺—C₃N₄; (b) a dispersion of 1 mg C₃N₄ in 0.054 μmol CoCl₂; and (c) 1 mg bare C₃N₄. In all three conditions, light intensity of 200 mW/cm² was applied for two hours. Negligible CO production was observed using bare C₃N₄. CO₂ reduction using a dispersion of CoCl₂ and C₃N₄ also generated significant amount of CO. However, Co²⁺—C₃N₄ complex produced five times more CO at the same cobalt loading compared to the dispersion of CoCl₂ and C₃N₄. This highlights the importance of cobalt loading processes, in which Co—N coordination likely occurred, in activating the Co²⁺ sites on C₃N₄.

The effect of cobalt loading on the photocatalytic activity of Co²⁺—C₃N₄ was also examined under the same experimental conditions. FIG. 16 shows amounts of CO produced when amounts of cobalt loadings in Co²⁺—C₃N₄ were varied. Results show significant CO production even at cobalt loadings lower than 0.010 μmol/mg of Co²⁺—C₃N₄. The amount of CO generated in CO₂ reduction increased linearly with cobalt loading until it reached 0.128 μmol/mg. Further increase in cobalt content beyond 0.128 μmol/mg resulted in a slight decrease in the amount of CO produced.

Quantum Yields and the Effects of Cobalt Loading on Quantum Yields

Quantum yields for photocatalytic CO₂ reduction were estimated based on the amounts of CO produced and the amounts of photons absorbed by the reaction solutions. The change in light intensity at 400 nm was measured after passing through a reaction solution with a cross section area of 4.95 cm². The following equation was used to calculate quantum yields because CO₂-to-CO conversion is a two-electron process,

Quantum Yield=2n(CO)/n(photon)

where n(CO) and n(photon) are the amounts of CO molecules produced and the number of photons absorbed, respectively.

Table 1 shows quantum yields for CO production after CO₂ reduction for 2 h using 1 mg Co²⁺—C₃N₄ with different cobalt loadings prepared in absence or presence of TEA. Light intensity was applied at 200 mW/cm².

Co loading CO produced Quantum yield (μmol/mg) (μmol) (%) No TEA 0.004 0.390 0.031 Samples prepared 0.004 0.292 0.023 in the presence 0.008 0.310 0.025 of TEA 0.033 0.482 0.039 0.054 0.595 0.048 0.128 1.056 0.084 0.206 1.002 0.081 0.345 0.936 0.075

The quantum yield was further optimized by varying the amounts of Co²⁺—C₃N₄ used as well as by varying the light intensity applied in photocatalysis using Co²⁺—C₃N₄ with a cobalt loading of 0.128 μmol/mg. Table 2 shows optimized quantum yields for CO production by reducing CO₂ for 2 h using Co²⁺—C₃N₄ with a cobalt loading of 0.128 μmol/mg.

Photocatalyst Mass Light Intensity Quantum Yield (mg) (mW/cm²) (%) 1 200 0.08 4 100 0.25 10 60 0.40 20 20 0.17

Quantum yields up to 0.40% were obtained for CO production using the synthesized materials.

Turnover Numbers and the Effects of Cobalt Loading on Turnover Numbers

The effect of cobalt loading on the photocatalytic activity of Co²⁺—C₃N₄ was also investigated by comparing turnover numbers (TONs) which were calculated based on the amount of product and the amount of cobalt present in the reaction suspension.

FIG. 17A shows turnover numbers for CO and H₂ production as a function of reaction time using a Co²⁺—C₃N₄ sample prepared in the absence of TEA. Results show that TON for CO formation, after reducing CO₂ for 2 h, is about 100. FIG. 17B, however, shows that TONs for CO formation drastically decreased when the cobalt loading was increased. This indicates the existence of single site catalysis at low cobalt loading. At higher cobalt loading, a significant portion of cobalt likely exists in the form of inactive cobalt oxides which do not contribute to catalytic reduction of CO₂.

The Co²⁺—C₃N₄ samples demonstrated excellent activity under visible-light irradiation (λ>420 nm). FIG. 18 shows that a TON greater than 200 was obtained after CO₂ reduction for 24 h using a Co²⁺—C₃N₄ sample. Isotopic studies, as shown by the insert in FIG. 18, clearly show that CO was produced as a result of CO₂ reduction, and that CO was not generated from any other source.

Photocatalytic CO₂ reduction using Co²⁺—C₃N₄ is quite selective towards CO production. FIG. 19 shows the percentage selectivity of Co²⁺—C₃N₄ catalyst for CO formation, and the product selectivity for CO formation remained independent of cobalt loading. That is, even at higher cobalt loading in Co²⁺—C₃N₄, the percentage selectivity for CO remain unchanged. This suggests that the inactive cobalt, may be deposited in the form of cobalt oxides at higher cobalt loading, does not take part or promote undesirable product formations. The open circle in FIG. 19 represents the Co²⁺—C₃N₄ formed in absence of TEA. Closed circles in FIG. 19 represent the Co²⁺—C₃N₄ formed in presence of TEA.

Stability of Co²⁺—C₃N₄

The stability of Co²⁺—C₃N₄ was also demonstrated as indicated by significant CO production using reused Co²⁺—C₃N₄ in FIG. 20. In order to test the reusability of Co²⁺—C₃N₄, 10 mg of Co²⁺—C₃N₄ with cobalt loading of 0.128 μmol/mg was dispersed in 4.0 mL of acetonitrile containing TEOA and irradiated with light intensity of 60 mW/cm² for 2 h (λ>350 nm). The resultant CO amount produced was recorded. After 2 h, the photocatalyst Co²⁺—C₃N₄ was collected by centrifugation, and rinsed with acetonitrile, and then was redispersed in a fresh 4.0 mL of acetonitrile containing TEOA. The photocatalysis was then repeated under the same conditions after purging with CO₂. FIG. 20 shows five cycles of photocatalysis using same Co²⁺—C₃N₄ sample where each cycle of catalysis lasted for two hours. It is interesting to note that Co²⁺—C₃N₄ is able to maintain its catalytic efficiency even after 5 cycles.

In addition to maintaining photocatalytic ability, Co²⁺—C₃N₄ did not exhibit any measured morphological changes after photocatalysis as shown in FIGS. 21A-B. FIG. 21A-B show SEM images of Co²⁺—C₃N₄ with a cobalt loading of 0.004 μmol/mg: (a) before and (b) after photocatalysis for 30 h. No morphological changes were observed in Co²⁺—C₃N₄ even after 30 h of photocatalysis. FIGS. 21C-D provide SEM images of Co²⁺—C₃N₄ with a cobalt loading of 0.128 mol/mg: (a) before and (b) after photocatalysis for five 2-hour cycles. Even after 5 cycles of photocatalysis, no morphological changes were observed in Co²⁺—C₃N₄. This confirms the photocatalytic and structural stability of Co²⁺—C₃N₄.

Confirmation of Model Having Metal Coordination at Edge Sites

Experiments were performed to confirm that metal-N coordination is responsible for CO₂-reduction activity using metal coordinated at edge sites of the model. C(x)-C₃N₄ materials with different amount of doped C were synthesized (with x=0, 5, 20, 55, 150, and 400 mg) and characterized with different techniques including UV-vis. The samples were synthesized from 99.6% urea and a small amount of dextrose as a carbon source. FIG. 22 illustrates the UV-vis spectra from 200 nm to 800 nm for each of the prepared samples. Increased absorbance in the visible spectra was observed for increased amounts of doped carbon.

The C(x)-C₃N₄ materials containing single Co²⁺ sites were tested in a photocatalytic CO₂ reduction. The photocatalysis conditions include 1 mg photocatalyst in 4.0 mL acetonitrile containing triethanolamine with a light intensity of 200 mW/cm². The sample Co²⁺ on C(20)-C₃N₄ (e.g., x=20) demonstrated the best activity among these materials as shown in FIG. 23.

The samples with CO₂-reduction activity were further characterized with EXAFS spectroscopy to confirm the presence of single Co²⁺ sites (peak absorption observed around 1.5 Å) as shown in FIG. 24. Together with activity, additional results obtained through analyzing EXAFS data support that the active Co^(n+) centers exist at edge sites on the C₃N₄ support. Table 3 below shows activity and EXAFS spectra fitting results for Co²⁺ on C(0)-C₃N₄ and C(20)-C₃N₄. Treatment with NH₃ was utilized to create more edge sites for Co²⁺ coordination. A TON for CO production of more than 800 was obtained after photocatalytic CO₂ reduction for 2 h using C²⁺ on NH₃-treated C(20)-C₃N₄.

Sample TON ^(a) d_(Co—N) ^(b) CN ^(c) Without NH₃ Co²⁺ on C(0)—C₃N₄ 4.4 2.08 ± 0.02 4.2 ± 0.9 treatment Co²⁺ on C(20)—C₃N₄ 61.6 2.08 ± 0.02 4.2 ± 1.1 With NH₃ Co²⁺ on C(0)—C₃N₄ 7.9 2.09 ± 0.02 4.5 ± 1.1 treatment Co²⁺ on C(20)—C₃N₄ 818.9 2.03 ± 0.05 4.2 ± 2.3 ^(a) Turnover numbers for CO production after CO₂ reduction for 2 hours; ^(b) Co—N distance in angstrom; ^(c) Co—N coordination number.

Based at least on these results, FIG. 25 illustrates a functional model of molecular catalysts that would be highly active in CO₂-reduction catalysis, where R=H or NH₂ and M²⁺=Co²⁺ or Ni²⁺. 

What is claimed is:
 1. A photocatalyst comprising: a graphitic carbon nitride providing nitrogen atoms for direct coordination with a transition metal ion in absence of additional ligands, the transition metal ion forming coordinate bonds with the nitrogen atoms at the edge sites of the graphitic carbon nitride, the nitrogen atoms forming a plane within the graphitic carbon nitride, and the transition metal ion being positioned outside the plane.
 2. The photocatalyst of claim 1, wherein the transition metal ion is Co²⁺.
 3. The photocatalyst of claim 2, wherein Co²⁺ is at a concentration between 0.004 and 0.430 μmol/mg of the photocatalyst.
 4. The photocatalyst of claim 2, wherein Co²⁺ is uniformly distributed on the graphitic carbon nitride.
 5. The photocatalyst of claim 2, wherein a molar ratio of Co²⁺ to cobalt oxide in the photocatalyst is greater than
 1000. 6. The photocatalyst of claim 1, wherein the graphitic carbon nitride is planar.
 7. The photocatalyst of claim 1, wherein the graphitic carbon nitride includes carbon doping.
 8. The photocatalyst of claim 1, wherein the transition metal ion is off the plane formed by the nitrogen atoms by a distance less than 0.5 Angstrom.
 9. The photocatalyst of claim 1, wherein the transition metal ion forms coordinate bonds with four nitrogen atoms at edge sites of the graphitic carbon nitride.
 10. The photocatalyst of claim 1, comprising earth-abundant elements.
 11. A method for manufacturing a photocatalyst, the method comprising: preparing a mixture of graphitic carbon nitride and a transition metal salt in a polar solvent; and forming single transition metal ion sites on the graphitic carbon nitride, the graphitic carbon nitride providing nitrogen atoms for direct coordination with the transition metal ion in absence of additional ligands, the transition metal ion forming coordinate bonds with the nitrogen atoms at the edge sites of the graphitic carbon nitride, the nitrogen atoms forming a plane within the graphitic carbon nitride, and the transition metal ion being positioned outside the plane.
 12. The method of claim 11, wherein the graphitic carbon nitride includes carbon doping with an optimal carbon content.
 13. The method of claim 11, wherein the transition metal ion is Co²⁺.
 14. The method of claim 13, wherein the complex has a Co²⁺ concentration between 0.004 and 0.430 μmol/mg of the complex.
 15. The method of claim 11, the mixture further comprising triethylamine.
 16. The method of claim 11, wherein the polar solvent comprises acetonitrile.
 17. The method of claim 11, wherein the polar solvent comprises a volume ratio of acetonitrile to triethylamine of from 100:1 to 150:1.
 18. The method of claim 11, wherein the transition metal ion salt comprises a cobalt halide.
 19. The method of claim 18, wherein the transition metal ion salt is cobalt dichloride.
 20. The method of claim 11, comprising heating the mixture. 